Role of redox reactions involving interfacial oxygen migration in ignition of nanolayered Al/a-Al2Ox/CuO energetic composite

Abstract

Thermite reactions of nanoparticles involve complex interplays between phase transition and heterogeneous atomic diffusion, especially at the early stage where the temperatures are relatively low and the systems remain as condensed phases. Moderate-temperature redox reactions among these condensed-phase reactants are investigated for a nanostructured 2.2 nm thick Al/CuO multilayer using ab initio molecular dynamics simulations. In order to achieve metastability, one amorphous alumina (a-Al2Ox) nanolayer of 0.6 nm is created between the Al and CuO layers. While the simulation is capable of predicting the energetic behaviors of the composite, focus is placed on investigating the species migration and reaction kinetics near these two interfaces, i.e., Al/a-Al2Ox and a-Al2Ox/CuyO. A set of redox reactions, possessing low activation barriers and a high exothermicity, are found to be critical to initiate the overall thermite reaction through generating localized hot spots within the a-Al2Ox layer and at its two interfaces, which has not been reported before. Driven by this exothermic process, migration of reactive species is promoted in condensed phases and a rate-limiting energy barrier (∼208.8 kJ•mol−1) at the Al/a-Al2Ox interface is overcome, which subsequently triggers a massive oxygen diffusion throughout the multilayer. Examination on chemical kinetics further reveals that the redox capacity of the a-Al2Ox layer determines the solid-state species migration during the ignition stage of the thermite reaction through two pathways: (1) the low-barrier and exothermic O migration in the a-Al2Ox layer and at the corresponding interfaces; (2) the migration-induced processes including decomposition of the CuO layer and melting of the Al. Contributions of the new simulation results to the existing reaction mechanisms are clarified.